This invention is concerned with the mounting of a combustion chamber in a gas turbine engine, and also to a combustion chamber mounting for supporting a combustion chamber within a gas turbine engine. More particularly, this invention is concerned with providing a combustion chamber mounting that will relieve stress resulting from differential thermal movement between a combustion chamber and surrounding structures to which the combustion chamber is attached.
In designing a gas turbine engine, it is desirable to increase the firing temperature to achieve greater thermal efficiency, and this entails the use of high temperature materials for the combustion chambers and associated transition ducts. High temperature alloys are commonly used to form a combustion chamber, but firing temperatures have risen above the highest operational temperature of these special alloys, necessitating either the provision of a cooling system (which increases manufacturing cost and reduces thermal efficiency), or forming the combustion chamber from a ceramic material capable of operating at the higher temperature.
A gas turbine combustion chamber is usually supported from a support structure, such as an engine or compressor casing, by at least one mounting. Under operating conditions, all of these components undergo thermal expansion which has to be accommodated by the design in order to avoid excessive stresses and strains.
Ceramic materials typically have a lower coefficient of thermal expansion than the materials forming the mounting and the associated support structure, with the consequence that substantial differential thermal expansion occurs between a ceramic combustion chamber and its mounting
Although a ceramic combustion chamber enables a higher firing temperature to be used, the stresses and strains, caused by thermal expansion and distortion of the combustion chamber and also by differential thermal expansion between the combustion chamber and its mounting, can result in failure of the combustion chamber by cracking due to the inherent brittleness of the ceramic material.
It is known from UK Patent Specification GB 1,476,414 for a ceramic combustion chamber to have a generally cylindrical side wall and a discharge end located, with freedom for axial expansion and contraction, within ducting for receiving the combustion products. An upstream end of the chamber is closed by an integral dome portion defining a central circular opening for an annular abutment which locates the fuel spray nozzle. This annular abutment is rigidly secured by studs to a combustion chamber cover and includes an annular flange which supports a slightly yieldable or resilient gasket positioned to react against the inner surface of the dome portion around the central circular opening. This inner surface of the dome portion is urged towards the rigid annular flange by a spring which reacts between structure rigidly mounted from the combustion chamber cover, and a rigid washer which bears against the outer surface of the dome portion around the central circular opening. There is no teaching concerning the accommodation of differential radial movement between the ceramic combustion chamber, the annular abutment and the combustion case cover. To the contrary, the spring urges the dome portion against the slightly yieldable or resilient gasket which is rigidly supported by the combustion case cover. The force exerted by this spring is clearly provided to cause the gasket to effect a seal between the rigidly mounted annular abutment and the dome portion of the combustion chamber.
This invention is based on the realization that stresses caused by thermal expansion and contraction of a combustion chamber, relative to the structures to which it is attached, can be relieved by permitting differential radial movement between the combustion chamber and its mounting, and by also permitting the combustion chamber to tilt relative to the structure from which it is supported. Such tilting can be caused by the thermal gradient between the cooler upstream end and the much hotter downstream end of the combustion chamber, and particularly by any thermal gradient transverse to the combustion chamber. Such transverse thermal gradients can be significant in a gas turbine having an annular array of combustion chambers, the combustion chamber walls adjacent the turbine axis being hotter.
According to one aspect of the invention a gas turbine has a combustion chamber secured to a mounting by a first attachment means arranged to accommodate differential radial movement between the combustion chamber and the mounting, the mounting being secured to a support structure by a second attachment means arranged to permit the combustion chamber to tilt relative to the support structure. In this manner the combustion chamber is positively located and supported by the mounting, which accommodates differential tilting and radial expansion and contraction of the combustion chamber, thereby avoiding the generation of excessive thermal stresses and strains in the material forming the combustion chamber. Whilst reduction of thermal stresses and strains is desirable for most combustion chambers, it is particularly beneficial when the combustion chamber is formed from a ceramic material.
The first attachment means preferably extends through an aperture in a wall of the combustion chamber and defines a pair of opposed surfaces shaped respectively to engage inner and outer surfaces of the combustion chamber wall adjacent the aperture, and the first attachment means also includes a biasing means operative to cause the opposed surfaces to grip the inner and outer surfaces of the combustion chamber wall with a force sufficient to secure the combustion chamber to the mounting whilst permitting differential radial movement between the combustion chamber wall and the opposed surfaces. With this arrangement, one of the opposed surfaces may be defined by a first member that is axially secured to the mounting, the other opposed surface being defined by a second member that is mounted for axial movement relative to the mounting, and the biasing means being arranged to react between the mounting and the second member. In this case an axial adjustment device may be arranged operatively between the first member and the mounting to enable the position of the said one opposed surface to be adjusted axially of the mounting, Preferably the first member is positioned within the combustion chamber.
The second attachment means preferably includes a second biasing device operative to oppose movement of the mounting relative to the support structure.
The mounting may be tubular and surrounds a fuel burner. In this case the support structure is preferably an air inlet guide vane communicating with the combustion chamber through the tubular mounting.
The mounting may include at least one duct for the passage of cooling air.
The combustion chamber is preferably formed of a ceramic material which may comprise woven continuous fibers embedded in a silicon carbide matrix. The surfaces of the first attachment means that are to contact the ceramic combustion chamber are preferably covered with an abradable metallic coating.
According to another aspect of the invention a combustion chamber mounting has:
first attachment means for securing a combustion chamber to the mounting, said first attachment means being capable of accommodating limited differential radial movement between the combustion chamber and the mounting, and
second attachment means for securing the mounting to a support structure, said second attachment means being capable of permitting the mounting, and hence the combustion chamber, to tilt a limited amount relative to the support structure.
Further aspects of the invention will be apparent from the following description and the appended claims.